The ferroelectric liquid crystals were discovered by R. B. Meyer, et al., in 1975. At a later time, it was discovered by Clark and Lagerwall that by applying a horizontal alignment treatment and injecting a ferroelectric liquid crystal to be sealed within a cell formed with a cell gap of around 1 .mu.m, a spiral structure of the ferroelectric liquid crystal disappeared. By the described discoveries, it became evident that the ferroelectric liquid crystal could be used as a display element. The described ferroelectric liquid crystal is known as a surface stabilized ferroelectric liquid crystal (SSFLC).
Initially, the ferroelectric liquid crystal was believed to be applicable to a display element without difficulties. However, after various researches and developments had been made, complicated structures and properties of the ferroelectric liquid crystal were found. In the meantime, the difficulties in controlling the display element became evident. With regard to such ferroelectric liquid crystal, the parameter to be focused was found to be a unique layered structure of a ferroelectric phase (SmC* phase).
As shown in FIG. 7, the ferroelectric liquid crystal was believed to have a "bookshelf structure" in which a layer L which constitutes the ferroelectric liquid crystal is formed perpendicular to the surface of substrates 51. However, after carrying out various researches, the ferroelectric liquid crystal was found to have a "Chevron structure" in which the layer L is bent at a central portion as shown in FIG. 8. Such layered structure resulted from compensating for a volume shrinkage occurred due to a tilt of a molecule M in the SmC* phase by the surface area of the layer L.
Thereafter, it was found that the Chevron structure is basically associated with zig-zag defects which are often observed in the SSFLC element. Further, as shown in FIG. 9, it was found that by utilizing a pre-tilt in which the molecule M in a vicinity of the substrates 51 was tilted by a predetermined angle with respect to the substrates 51, a C1 orientation and a C2 orientation having different bending directions respectively of the layer L would appear as an alignment state.
In the case of adopting the ferroelectric liquid crystal for the display element, it is required to form a mono-domain by aligning liquid crystal molecules in a uniaxial direction in the entire display element. Known methods for such alignment include: a magnetic field alignment method, a temperature gradient method, a rubbing method, etc.
In the magnetic field alignment method, after heating the liquid crystal material to show an isotropic phase under an applied magnetic field, the liquid crystal is gradually cooled off to the liquid crystal phase. In the temperature gradient method, a liquid crystal phase is developed from the cut surface at a gradient temperature utilizing an alignment control force of the cut surface of a polymer spacer. In the rubbing treatment, the liquid crystal is aligned in a uniaxial direction by rubbing with cloth the alignment film in the uniaxial direction. This rubbing treatment is industrially the most effective.
When obtaining a mono-domain by re-aligning the display element which has been aligned by the known rubbing treatment, the method of gradually cooling off the display element which is heat treated until it shows an isotropic phase is generally adopted.
In the case of adopting the ferroelectric liquid crystal element, either one of the described C1 orientation and C2 orientation is adopted. This is because, when the described orientations of two kinds are mixed, zig-zag defects generate on the interface between them, which significantly lower the quality of the display element. Therefore, it is critical to control the C1 orientation and C2 orientation to attain an improved display quality. Additionally, as the C2 orientation has beneficial features of its stability at low temperature, and a short response time over the C1 orientation, the C2 orientation is more preferable than the C1 orientation.
Conventionally, to attain a uniform C2 orientation on an entire surface of the display element, the alignment is typically controlled by adjusting the combination of materials for a liquid crystal and an alignment film, or the rubbing conditions. The realignment is also typically achieved by adjusting the temperature or by the electric field treatment.
However, even if a liquid crystal material and an alignment film material showing desirable characteristics respectively are combined, if a combination thereof is not appropriate, the C2 orientation is hardly achieved by the alignment achieved by the rubbing method generally used. In such case, due to restrictions on an alignment, desirable properties of the materials for the liquid crystal and the alignment film cannot be fully appreciated.
As to the rubbing conditions, by increasing the rubbing strength, the C2 orientation appears. On the contrary, many scratches are formed on the display--element resulting from the strong rubbing treatment, thereby presenting the problem that the display quality suffers.
The re-alignment treatment does not enable a C2 orientation to always appear.
The described deficiencies of the conventional method have led to the need for development of a desirable method of forming a C2 orientation for sure on an entire surface of a display element without being affected by a combination of materials for a liquid crystal and an alignment film, rubbing conditions, etc.